Thick film conductors are widely used as a means of interconnecting various passive and active devices for hybrid microcircuits and resistor networks. Utility as a general purpose conductor requires certain performance attributes such as conductivity, solderability, solder leach resistance, compatibility with other circuit components and ability to be processed under a wide range of conditions. Inherent in the usefulness of thick film conductors is the cost of materials in the composition. It is extremely advantageous to reduce the cost without significantly changing the performance characteristics.
Thick film conductors are comprised of conductive metal and inorganic binder, both of which are in finely divided form and are dispersed in an organic medium. The conductive metal is ordinarily gold, palladium, silver, platinum or mixtures and alloys thereof, the choice of which depends upon the particular combination of performance characteristics which are sought, e.g., resistivity, solderability, solder leach resistance, migration resistance, adhesion and the like.
In the current economic climate in which noble metals have experienced substantial fluctuations in price, it is especially attractive from a business viewpoint to substitute less expensive base metals as the conductive metal in thick film conductor compositions. Several base metals have been proposed and used with mixed success as the conductive phase for thick film conductors. Among these the most important is copper.
When copper conductors are used in microcircuits, they are frequently subjected to quite rigorous conditions during fabrication. For example, in a typical application, the copper-containing composition is printed on a substrate, dried and fired in a nitrogen atmosphere at 900.degree. C. Then a pattern of resistor material is printed in proper registry atop the conductor layer and the copper-containing composition and overlying layer of resistor material are fired at about the same temperature in nitrogen to effect sintering of the resistor material. Following this, an overglaze may be applied and the entire assemblage is fired in nitrogen once again to sinter the overglaze material. When this is completed, leads are soldered onto the conductive layer. Thus, in this typical situation, the copper is subjected to as many as three high temperature firings and, in some fabrication, the copper-containing layer may be subjected to as many as ten such firings.
During such refirings the copper conductor loses solderability and bondability because of copper oxidation and migration of glass binder to the surface of the copper containing layer. Whenever the problem of bondability has risen with respect to noble metal conductors which are fired in air rather than nitrogen, one successful solution to the problem has been to overprint and fire a patterned thin layer of fritless gold onto the problem substrate in the area where the bonding is to take place. Thus, when components are attached to the conductor, they are bonded in the gold-overprinted areas. However, it has been shown not to be feasible to use gold as an overprint for copper because the metals form an undersirable alloy during firing. Furthermore, when fritless copper overprinting has been attempted in nitrogen-fired copper conductor systems, the overprinted copper layer rapidly becomes oxidized on firing and thus is rendered less solderable and bondable than the underlying layer.
Thus there remains a substantial need for an effective overprint composition which can be used in nitrogen fired copper conductor systems.